Respiratory physiology

Question 1

What are the two subsystems of the mammalian respiratory system?

Answer 1

External respiratory system: lungs and the pulmonary circulation
internal respiratory system consisting of the tissue cells

Question 2

What is really important for effective gas exchange?

Answer 2

Ventilation and perfusion must be matched

Question 3

In terms of ventilation what is highly variable and what remains fairly constant?

Answer 3

rate of ventilation and tidal volume are highly variable between healthy individuals

• Adult males: 10-18 breaths per min and tidal volumes of 350-900 ml
• Adult females: 10-18 breaths per min and tidal volumes of 200-650 ml

Rate of oxygen uptake at rest is relatively constant
Adult males: 240ml/min
Adult females: 200ml/min

Question 4

What does it mean by anatomical dead space?

Answer 4

Not all of the air inspired during relaxed ventilation reaches the respiring airways - remainder says in the mouth, pharynx and large conducting airways

Dead space ventilation = dead space volume X ventilation frequency - therefore dead space ventilation increases as breathing gets faster

Question 5

What is the physiological importance of dead-space?

Answer 5

sum of the anatomical dead space and the alveolar dead space

alveolar dead space= unperfused alveoli and a proportion that are only partially perfused

defined as “part of tidal volume that does not participate in gaseous exchange”

Question 6

What are the functions of the blood-gas barrier?

Answer 6

Prevents air bubbles forming in the blood and the blood leaking into the alveoli

Permeable to oxygen, carbon dioxide and other gases

Question 7

What does the blood-gas barrier comprise of?

Answer 7

type 1 pneumocytes,

the endothelial cells and the basement membrane of the two cells

Question 8

What is the ventilation perfusion rate and how is it calculated?

Answer 8

V/Q = 0.8
Every minute, an average of 4L of air enters the alveoli and round 5L of blood passes through the pulmonary capillaries in healthy adults

Question 9

Why is the ventilation/perfusion ratio in the lungs not evenly distributed?

Answer 9

Wide range of ratios from one extreme to another

• no blood perfuses a well-ventilated alveolus: V/Q = >1
• no air ventilated a well-perfused alveolus: V/Q = 0

Due to gravity perfusion is greater at the base of the lungs and ventilation is greatest at the apex of the lungs

It is often compromised in respiratory disease

Question 10

How is optimal gas exchange achieved by the pulmonary circulation?

Answer 10

Effective ventilation matched to effective perfusion

A balance between oxygen uptake and CO2 excretion and work of breathing

Question 11

What is Henry’s law?

Answer 11

the degree to which a gas dissolves in a liquid (blood) is directly proportional to the partial pressure of the gas above (alveolus)

Question 12

What is Fick’s law?

Answer 12

the rate of diffusion is proportional to the diffusion constant, surface area and pressure gradient and inversely proportional to the tissue thickness.

= dV/dt = D x (P1-P2)

Question 13

What are the two forms of Hb?

Answer 13

taut (tense) form and relaxed form

Question 14

What factors favour taut form?

Answer 14

low pH
higher CO2
higher temperature

it causes Hb to have a lower affinity for oxygen causing it to release oxygen

Question 15

What factors favour the relaxed form of Hb?

Answer 15

higher pH
lower CO2
lower temp

binds oxygen more strongly in the alveolar capillaries

Question 16

Where is 2,3-BPG present and what does it do?

Answer 16

present in human erythrocytes
binds with greater affinity to deoxyghaemoglobin decreasing its affinity for oxygen

therefore promotes the release of remaining oxygen molecules

Question 17

Which way does the Bohr shift go in terms of the oxygen dissociation curve and what promotes it?

Answer 17

Shifts the sigmoidal curve to the right

• increased temperature
• increased 2,3-BPG
• decreased pH

Question 18

Which way does the Haldane shift go in terms of the oxygen dissociation curve and what promotes it?

Answer 18

Shifts the sigmoidal curve to the left

• decreased temperature
• decreased 2,3-BPG
• increased pH

Question 19

What is aerobic respiration?

Answer 19

Chemical process by which organisms extract energy from food
= oxidation reaction that produces chemical energy from glucose and oxygen
C6H1206 + 602 goes to 6H20 + 6CO2 + ENERGY

Question 20

How is CO2 transported in the blood?

Answer 20

dissolved in the plasma (5%)
bound to Hb and plasma proteins (about 10%)
majority is transported as bicarbonate ions (HCO3-)

Question 21

What is the equation for the formation of bicarbonate ions?

Answer 21

CO2 + H20 catalysed by carbonic anhydrase to H2CO3 which goes to HCO3- and H+

Question 22

What are the two main buffer systems and what do they do?

Answer 22

The lungs removal CO2 produced by metabolism

The kidneys deal with ingested dietary acids

Buffer systems help the kidney and lungs maintain pH within narrow limits

Question 23

What is the principle buffer system?

Answer 23

Bicarbonate buffer, though phosphate and proteins are also involved

Question 24

What is the Henderson-Haslebalch equation?

Answer 24

Modified version to relate to the pH of blood

pH = pKa(H2CO3) + log([HCO3-]/[H2CO3])

pKa(H2CO3)= -ve log of the acid dissociation constant of carbonic acid = 6.4

Question 25

What are the different respiratory gas analysis approaches?

Answer 25

may analyse blood gases, exhaled gases or non-invasive methods via the skin surface

Question 26

What are the invasive methods of respiratory gas analysis?

Answer 26

Arterial blood gas sampling (ABG) | Capillary Blood Gas sampling (CBG)

Question 27

What are the non-invasive methods of respiratory gas analysis?

Answer 27

Pulse Oximetry | Transcutaneous monitoring

Question 28

Describe the ABG

Answer 28

Gold standard method Direct from the artery (usually radial artery) No potential for contamination with the air Can make duplicate measurements from one sample or serial measurements with an arterial line Can be very painful

Question 29

Describe the CBG

Answer 29

Useful and under-used alternative to ABGs Sampled from the earlobe so its not as painful and invasive Site is prepared with vasodilator cream (deep heat) to arterialise the blood Blood must free-flow into the tube and not be forced Although outcome measures are not exactly the same studies have shown no significant difference

Question 30

What are the outcome measures and their normal values for ABG/CBG?

Answer 30

``` PaO2: 10-13.5kPa PaCO2: 4.8-6 kPa pH: 7.35-7.45 HCO3-: 23-27mmol/L Base excess (surplus alkali): -3 to 3 mmol/L Oxygen saturation: 96-99% ```

Question 31

What is the alveolar gas equation?

Answer 31

allows the calculation of the alveolar partial pressure of oxygen from data the is practically measurable: = FiO2 (Patm -PH2O) - PaCO2/RER ........ = 0.21(101-6.25)-PaCO2/0.8

Question 32

What is the alveolar-arterial difference?

Answer 32

A-a gradient is a measure of the difference between alveolar and arterial oxygen Normally 0.5-1.5kPa

Question 33

What does it mean if the A-A gradient increases, >1.5kPa?

Answer 33

means a low PaCO2, low PaO2 or both e.g. patient breathing hard to try and achieve normal oxygenation, a patient breathing normally and attaining low oxygenation

Question 34

What is a pulse oximetry?

Answer 34

small, portable device capable of measuring oxygen saturation and heart rate Not as accurate as full blood gas analysis but.. Simple, useful and non-invasive Used during spot-checks (within community) or continual monitoring (overnight sleep studies)

Question 35

What is transcutaneous monitoring?

Answer 35

Monitor partial pressures of oxygen (TcO2) and carbon dioxide (TcCO2) Measured via a sensor attached to the skin (upper forearm or below collar bone) Useful for monitoring patients with more sever respiratory compromise (where CO2 may be raised)

Question 36

What happens in respiratory failure?

Answer 36

impaired pulmonary ventilation and/or perfusion effects gas exchange and can lead to altered respiratory blood gas composition = breathlessness, decreased exercise tolerance, impaired cognition and even loss of consciousness

Question 37

What are the different types of respiratory failure?

Answer 37

Type I Acute type II Chronic type II Metabolic acidosis/alkalosis

Question 38

What is type I respiratory failure?

Answer 38

PaO2 < 8kPa (Severe hypoxia) Mild hypoxia may not be pathologically significant More severe hypoxia is a major cause of organ dysfunction and patient death Long term oxygen therapy can be indicated

Question 39

What is nocturnal hypoxia?

Answer 39

Established mild hypoxia worsens during sleep due to suppression of respiratory system Nocturnal oxygen often prescribed

Question 40

When can intermittent hypoxia occur?

Answer 40

can result from sleep-related breathing disorders - e.g. obstructive sleep apnoea

Question 41

What is acute type II respiratory failure?

Answer 41

Hypoxia in conjunction with hypercapnia - increased CO2 leads to increased H+ = decreased pH Kidneys have not yet compensated (base excess will be normal) Often occurs during an exacerbation of respiratory disease (e.g. chest infection in COPD)

Question 42

What is chronic type II RF?

Answer 42

Abnormal pH If increased CO2 persists the kidneys produce more alkali to compensate which returns the pH to normal (3-5 days) Common in people with severe respiratory disorders (COPD, khphoscoliosis) and long-term ventilatory support may be required

Question 43

What is the spirometry lung function test?

Answer 43

``` Spiro=breath metry=measurement Assess pulmonary ventilation Simple and most common Versatile and informative that often correlated well to disease severity ```

Question 44

What measures do you gain from a spirometry test?

Answer 44

``` Vital capacity Forced vital capacity (FVC) Forced expired volume in 1 second (FEV1) FEV1/FVC ratio % Peak expiratory flow (L/s) ```

Question 45

What is the spirometry procedure?

Answer 45

Full inspiration then immediate full expiration at relaxed speed (VC) or at max speed (FVC) Inspiration at relaxed speed (IVC) OR at max speed (FIVC) is optional

Question 46

What is the most useful aspect of determining lung volumes?

Answer 46

it provides information on the functional status of the lungs - not hugely information -useful with severe airflow obstructions

Question 47

What is FRC, ERV, RV, EVC & IVC, and TLC?

Answer 47

``` FRC: functional residual capacity (L) ERV: expiratory reserve volume (L) RV: residual volume (L) EVC: expiratory vital capacity IVC: inspiratory vital capacity TLC: total lung capacity (L) ```

Question 48

What tests can be used to measure lung volume?

Answer 48

Gas dilution techniques - helium wash in - nitrogen wash out Body plethysmography

Question 49

What is the helium dilution test?

Answer 49

Known vol and conc of helium breathed tidally diluted in lungs until equilibrium is met degree to which the gas is diluted will be proportional to the FRC

Question 50

What is the nitrogen wash out test?

Answer 50

Nitrogen = 78% gas in lungs. FRC can be determined by calculating the total volume of nitrogen in the lungs Breath in 100% O2 to gradually wash out nitrogen Expired gas collected in spirometer and nitrogen conc is recorded breath by breath

Question 51

What is the single breath gas transfer test?

Answer 51

Most common test Quantifies oxygen uptake by respiratory system Using CO as it binds in the same manner as O2 to Hb but at a much greater affinity (210x)

Question 52

What is the TLco, VAeff and Kco?

Answer 52

TLco: transfer factor of the lungs for CO - estimate of overall efficiency of oxygen uptake (also dependent on Hb) VAeff: effective alveolar volume - Surface area over which gas transfer occurs Kco: transfer coefficient (TL/VA) - estimate of efficiency of O2 uptake at alveolar level